Coating compositions are often divided into thermoplastic and thermoset. Thermoplastic coating compositions utilize high molecular weight polymers dispersed in an organic or aqueous solvent. After the coating composition is applied to a substrate, the solvent is removed and the polymers set up to form a film. Thermoset or curable coating compositions utilize two components that are reactive with one another under certain cure conditions. The reactive groups on these components are referred to as `functional groups`. After the composition containing these components is applied, the coated substrate is subjected to cure conditions, causing the functional groups to react and form a cured film of a crosslinked matrix.
Various combinations of functional groups have been used in the art for curable coating compositions. One widely used combination utilizes OH-functional groups and the alkylol groups on aminoplast resins. These functional groups react to form an ether bridges in the crosslinked coating. Another combination utilizes acid functional groups and epoxy functional groups, which react to form ester bridges in the crosslinked coating.
Curable coating compositions that cure via the formation of urethane bridges in the crosslinked coating have also been used in the art. Urethane bonds are often desirable for their durability, resistance to attack by various agents in the environment, impact resistance, and other physical properties such as stress release. Urethane bridges can be formed by various combinations of functional groups, such as OH functional groups and isocyanate functional groups, carbamate groups and aminoplast resins, or cyclic carbonate groups and amino groups.
In many coating applications, it is often necessary to apply an additional coating on top of an already-cured coating. This may be done to achieve certain visual or physical properties, or it may be necessary to repair areas where the cured coating has been damaged or where coating defects exist. Areas in need of repair are usually identified by visual inspection of the coated surface, which may be aided by well-known mechanical or electronic inspection aids. In these cases, it is important that the coating applied on top of the cured coating have adequate adhesion to the cured coating. Even when the cured coating is sanded prior to application of the additional coating, the adhesion factor is still a concern with regard to overspray areas onto unsanded coating surfaces.
Intercoat adhesion can be of particular concern with regard to cured urethane coatings. Accordingly, the present invention is directed toward such a curable coating composition having good intercoat adhesion properties.